A HISTORY OF SCIENCE 



life must be re-estimated. Until the discovery of 

 radium it seemed definitely determined that the earth 

 was gradually cooling, and would continue to cool, un 

 til, like the moon, it would become too cold to support 

 any kind of vegetable or animal life whatever. But 

 recent estimates of the amount of radio-active matter 

 in the earth and atmosphere, and the amount of heat 

 constantly given off from this source, seem to indicate 

 that the loss of heat is (for the moment) about evenly 

 balanced by the heat given out by radio-active matter. 

 Thus at the beginning of the new century we see 

 the phenomenon of a single discovery in science com- 

 pletely overturning certain carefully worked out calcu- 

 lations, although not changing the great principles in- 

 volved. It is but the repetition of the revolutionary 

 changes that occur at intervals in the history of 

 science, a simple discovery setting at naught some of 

 the most careful calculations of a generation. 



